Many of today's computing applications such as cellular phones, digital cameras, and personal computers, use nonvolatile memories to store data or code. Nonvolatility is advantageous because it allows the computing system to retain its data and code even when power is removed from the computing system. Thus if the system is turned off or if there is a power failure, there is no loss of code or data. Such nonvolatile memories include Read-Only Memory (ROMs), Electrically Programmable Read-Only Memory (EPROMs), Electrically Erasable Programmable Read-Only Memory (EEPROMs), and flash Electrically Erasable Programmable Read-Only Memory (flash EEPROMs or flash memory). However, only EEPROMs and flash memory are electrically re-writable within the host system, making these memories more flexible and easier to use.
Nonvolatile semiconductor memory devices are fundamental building blocks in computer system designs. One such nonvolatile memory device is flash memory. Flash memory can be programmed by the user, and once programmed, the flash memory retains its data until the memory is erased. Electrical erasure of the flash memory erases the contents of the memory of the device in one relatively rapid operation. The flash memory may then be programmed with new code or data. The primary mechanism by which data is store in flash memory is a flash memory cell. Accordingly, outputs of a flash memory device are typically associated with an array of flash cells that is arranged into rows and columns such that each flash cell in the array is uniquely addressable.
A flash EEPROM memory device (cell) is a floating gate MOS field effect transistor having a drain region, a source region, a floating gate, and a control gate. Conductors are connected to each drain, source, and control gate for applying signals to the transistor. A flash EEPROM cell is capable of functioning in the manner of a normal EPROM cell and will retain a programmed value when power is removed from the circuitry. A flash EEPROM cell may typically be used to store a one or zero condition. If multilevel cell (MLC) technology is used, multiple bits of data more be stored in each flash EEPROM cell. Unlike a typical EPROM cell, a flash EEPROM cell is electrically erasable in place and does not need to be removed and diffused with ultraviolet to accomplish erasure of the memory cells.
Arrays of such flash EEPROM memory cells have been used in computers and similar circuitry as both read only memory and as long term storage which may be both read and written. These cells require accurate values of voltage be furnished in order to accomplish programming and reading of the devices. Arrays of flash EEPROM memory devices are typically used for long term storage in portable computers where their lightweight and rapid programming ability offer distinct advantages offer electromechanical hard disk drives. However, the tendency has been to reduce the power requirements of such portable computers to make the computers lighter and to increase the length of use between recharging. This has required that the voltage potentials available to program the flash memory arrays be reduced. It is now necessary to generate such precision voltage potentials within the circuitry for controlling the flash EEPROM memory array. High precision voltages are also useful for providing faster read times and better reliability of the flash EEPROM cells.
A number of the electronic systems that use flash memories are small portable devices that rely on batteries for power. Consequently, it is desirable to increase the battery life of these devices by reducing power consumption. The power consumption is reduced in many of these portable electronic devices by operating specific circuits or components in a lower power standby mode during periods when these components are not required. Typically, this low power standby mode would reduce the overall current consumed by the component.
The charge pumps and reference voltage generating circuits in flash memory devices are mostly turned off in a standby/deep power-down mode to save power consumption. But the pumps and reference voltage generator need to be turned on periodically to charge the positive and negative nodes towards the read level voltages. This process is to enable the flash device to enter the `read mode` as soon as there is a read request while the flash device is in standby/deep power-down mode. When these circuits are turned off, the positive and negative read nodes get discharged due to leakage. The leakage, and hence the read nodes discharging, is positively related to temperature. Hence, there is a need to turn on the pumps and reference voltage generators more often at high temperatures to compensate for the higher discharge rate.
The enabling circuits that have been used so far, such as the medium frequency oscillator (MFO) which periodically turn on the pumps and reference generators in the standby/deep power-down mode, have had a negative frequency correlation with temperature. Hence, these enabling circuits were designed to cover the worst case discharge rate at high temperatures. As a result, the MFO would turn on the charge pumps and the reference generator for recharging nodes more often at low temperatures than necessary despite lower the discharge rates. Hence, the standby/deep power-down power consumption of the flash device was high and unoptimized.